In semiconductor fabrication, various lithographic processes are extensively used in the course of defining devices and circuit patterns. Depending on the size of the features to be defined, different optical lithographic processes may be used. In general, as the patterns become smaller, shorter wavelengths are utilized. In extreme ultraviolet lithography (“EUVL”) a wavelength of about 13.5 nm is frequently used. In EUVL, a pattern present on a photomask or reticle may be transferred to a layer sensitive to extreme ultraviolet (“EUV”) radiation by illuminating the reticle with EUV radiation. The EUV light is modulated by the reticle pattern and imaged onto a photoresist-coated wafer.
In conventional lithography, a pellicle is commonly placed above the reticle to protect the reticle from contamination during handling and exposure etcetera. The pellicle will thus protect the reticle from unwanted particles which otherwise could negatively impact the fidelity of the pattern transfer to the wafer. As the pellicle remains above the reticle during exposure, there are stringent requirements on the pellicle in terms of absorption, durability, and particle shielding capabilities, etc.
When it comes to EUVL it has been challenging to find suitable pellicle candidates. Conventional deep ultraviolet (“DUV”) pellicles typically provide excessive absorption of extreme ultraviolet light even for very thin material thicknesses. Further, the high energy of the extreme ultraviolet light combined with the scanner environment is prone to damaging the material of the pellicle membrane. Thus, it has proved troublesome to identify pellicle designs that are compatible with EUVL.